1. Field of the Invention
The present invention relates to a non-contact digitizing method, and more specifically, to a non-contact digitizing method for creating a tracing data of a configuration of a three-dimensional model by controlling the attitude of a tracer head and tracing the configuration of the model while not in contact therewith,
2. Description of the Related Art
Recently, a non-contact trace control apparatus has been developed to trace a configuration of a model by using a non-contact distance sensing unit, An optical distance sensing unit is used as this non-contact distance sensing unit and is fixed to the extreme end of a tracer head, and a tracing is carried out by sensing a distance to a surface of a model, This tracing method eliminates any possibility of damage to a model, and accordingly, can be applied to a model composed of a soft material, and thus the applications for which a trace machining can be utilized are expected to increase,
In general, a function for tracing a configuration of a model, sensing data of the Locus of the traced configuration at all times, and automatically outputting the data to an NC tape or the like is referred to as a digitizing method, A non-contact digitizing method enables digitizing data to be calculated without damage to a model surface.
In this connection, when a portion of a model having a large angle of inclination is traced by a conventional non-contact trace control apparatus, a problem arises in that a tracing accuracy of this portion is poor, and thus digitizing data having a pinpoint accuracy cannot be obtained. For example, when a surface of a model almost perpendicular to the surface on which the model is installed is traced, and the measurement axis of a tracer head is almost parallel with the surface of the model, a circular spot on the surface of the model is enlarged and distorted to become an ova I shape and thus the distance sensing resolution is Lowered. Therefore, to perform a non-contact digitizing with a high accuracy, a configuration of a model must be traced by using a tracer head able to be inclined in a direction optimum to a model surface, instead of a tracer head fixed in the direction (Z-axis direction) perpendicular to a plane on which the model is installed.
To solve the above problem, the inventors have U.S. Pat. No. 5,140,230, wherein a tracer head is provided with two non-contact distance sensing units, a normal vector on a model surface is determined based on a measured value obtained by sampling tracing data from the tracer head, and the tracer head is controlled in the direction of a projection obtained by projecting the normal vector onto a predetermined plane.
The invention disclosed in the preceding patent application relates to a non-contact trace control apparatus whereby three points are selected from a plurality of measured points, two surface vectors are determined from these three points, and the outer product of these surface vectors is calculated to determine the normal vector. Since, however, an order in which the surface vectors are multiplied is not fixed, a problem arises in that, when the order is reversed, two normal vectors facing in completely opposite directions are determined.
At this time, when a normal vector N having a positive value NZ of a Z-axis component is selected, this problem usually can be solved. Nevertheless, a normal vector N having a negative value NZ may be selected as a correct normal vector, particularly when a configuration of a model having a vertical or almost vertical surface is traced, due to a measurement error caused by a non-contact distance sensing mechanism or a calculation error caused by a calculating mechanism. Consequently, a normal vector that is unsuitable as a normal vector of a model surface is selected in the above non-contact trace control apparatus, the measurement axis of the tracer head is rotated by 180.degree., and thus a problem arises in that the tracer head cannot perform the required measurement or interferes with the model surface.